U.S. patent number 9,691,557 [Application Number 14/727,349] was granted by the patent office on 2017-06-27 for free-standing hybrid nanomembrane as energy storage electrode and the fabrication method thereof.
This patent grant is currently assigned to IUCF-HYU (Industry-University Cooperation Foundation Hanyang University). The grantee listed for this patent is IUCF-HYU (Industry-University Cooperation Foundation Hanyang University). Invention is credited to Seon Jeong Kim, Shi Hyeong Kim, Jae Ah Lee, Min Kyoon Shin.
United States Patent |
9,691,557 |
Kim , et al. |
June 27, 2017 |
Free-standing hybrid nanomembrane as energy storage electrode and
the fabrication method thereof
Abstract
Disclosed is a free-standing hybrid nanomembrane capable of
energy storage. The free-standing hybrid nanomembrane includes
carbon nanotube sheets and a conducting polymer coated on the
carbon nanotube sheets. The carbon nanotube sheets are densified
sheets formed by evaporating an alcohol from carbon nanotube
aerogel sheets. The conducting polymer is coated on the carbon
nanotube sheets by vapor phase polymerization. Further disclosed is
a method for fabricating the free-standing hybrid nanomembrane.
Inventors: |
Kim; Seon Jeong (Seoul,
KR), Kim; Shi Hyeong (Seoul, KR), Shin; Min
Kyoon (Seoul, KR), Lee; Jae Ah (Seoul,
KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
IUCF-HYU (Industry-University Cooperation Foundation Hanyang
University) |
Seoul |
N/A |
KR |
|
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Assignee: |
IUCF-HYU (Industry-University
Cooperation Foundation Hanyang University) (Seoul,
KR)
|
Family
ID: |
50880717 |
Appl.
No.: |
14/727,349 |
Filed: |
June 1, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20150294800 A1 |
Oct 15, 2015 |
|
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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13710796 |
Dec 11, 2012 |
9251968 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01G
11/36 (20130101); H01G 11/86 (20130101); H01G
11/48 (20130101); Y02E 60/13 (20130101); B82Y
30/00 (20130101) |
Current International
Class: |
H01G
11/36 (20130101); H01G 11/48 (20130101); H01G
11/86 (20130101); B82Y 30/00 (20110101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Cheng, W., et al., "Freestanding ultrathin nano-membranes via
self-assembly;" nanotoday, vol. 4, No. 6; dated Dec. 2009; pp.
482-493. cited by applicant .
Elbaccouch, M., et al.; "Microstructural analysis of
doped-strontium cerate thin film membranes fabricated via polymer
precursor technique;" Solid State Ionics, vol. 178, No. 1-2; pp.
19-28; dated Jan. 2007. cited by applicant .
Fukao, N., et al.; "Automatic Spray-LBL Machine Based on in-Situ
QCM Monitoring;" Macromolecules, vol. 44, No. 8; pp. 2964-2969;
dated Apr. 2011. cited by applicant .
Gonzalez-Perez, A., et al.; "Biometric triblock copolymer
membranes: from aqueous solutions to solid supports;" Soft Matter,
vol. 7, No. 3; pp. 1129-1138; dated Dec. 2010. cited by applicant
.
Jiang, C., et al.; "Self-recovery of stressed nanomembranes;"
Applied Physics Letters, vol. 86, No. 12; p. 121912. cited by
applicant .
Kang, T., et al., "Ultra-thin and Conductive Nanomembrane Arrays
for Nanomechanical Transducers;" Advanced Materials, vol. 20, No.
16; pp. 3131-3137. cited by applicant .
Singamaneni, S., et al.; "Robust Fluorescent Response of
Micropatterned Multilayered Films;" Journal of Macromolecular
Science, vol. 46, No. 1; pp. 7-19; dated Apr. 2007; retrieved on
Mar. 5, 2013 from
<http://www.google.com/search?q=Robust+Fluorescent+Response+of+Micropa-
tterned+Multilayered+Films&rls=com.microsoft:en-us&ie=UTF-8&oe=UTF-8&start-
Index=&startPage=1>. cited by applicant .
Zhao, D., et al.; "Multiphase Assembly of Mesoporous-Macroporous
Membranes;" Chemistry of Materials, vol. 11, No. 5; pp. 1174-1178;
dated Apr. 1999; retrieved on Mar. 5, 2013 from
<www.chemengr.ucsb.edu/.about.ceweb/faculty/bradc/pdfs/19.pdf>.
cited by applicant.
|
Primary Examiner: Ferguson; Dion R
Attorney, Agent or Firm: Alston & Bird LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. application Ser. No.
13/710,796, filed Dec. 11, 2012, which is hereby incorporated
herein in its entirety by reference.
Claims
What is claimed is:
1. A method for fabricating a hybrid nanomembrane in which a
conducting polymer is coated on carbon nanotube sheets, the method
comprising 1) allowing an oxidant solution comprising Fe(III)PTS,
pyridine and butanol to flow on carbon nanotube sheets, and
completely evaporating the butanol, and 2) adding a monomer for a
conducting polymer polymerization to the carbon nanotube sheets
stained with the oxidant solution free from butanol, followed by
vapor phase polymerization in a sealed state.
2. The method according to claim 1, further comprising 3) rinsing
the hybrid nanomembrane to remove unreacted oxidants after step
2).
3. The method according to claim 1, wherein the carbon nanotube
sheets are densified sheets formed by evaporating an alcohol from
carbon nanotube aerogel sheets.
4. The method according to claim 1, wherein the monomer for the
conducting polymer is ethylenedioxythiophene (EDOT).
5. The method according to claim 1, wherein the concentration of
the Fe(III)PTS in the oxidant solution is from 6 to 20% by weight,
based on the weight of the butanol.
6. The method according to claim 1, wherein, in step 1), the
alcohol is evaporated at 40 to 60.degree. C.
7. The method according to claim 1, wherein, in step 2), the
polymerization is carried out at around 60.degree. C.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nanomembrane and a fabrication
method thereof. More specifically, the present invention relates to
a free-standing hybrid nanomembrane capable of energy storage in
which a conducting polymer is coated on densified carbon nanotube
sheets, and a method for fabricating the free-standing hybrid
nanomembrane.
2. Description of the Related Art
Freely suspended ultrathin (thickness <100 nm) membranes promise
applicability of many types of sensors, actuators, optical devices,
fuel cells, scaffolds for the organization of nanoparticles, and
separation of biological macromolecules. Several approaches have
been carried out for the fabrication of free-standing nanomembranes
from polymers and/or from inorganic materials: spin casting of
films, (Solid-state ionics 2007, 178, 19) layer-by-layer assembly
of polyelectrolytemulti-layers, (Macromolecules 2011, 44, 2966)
crosslinking of self-assembled monolayers, (Nano today 2009, 4,
4829) and assembly of triblockcopolymers (Chem. Mater. 1999, 11,
1174, Soft Matter 2011, 7, 1129). For mechanically robust and
electrically conductive nanomembranes, nanoparticles such as gold
nanoparticles or carbon nanotubes have been embedded in the films
(Macromolecular science, part B: Physics, 2007, 46, 7, Adv. Mater.
2008, 20, 3131).
However, most methods are often time consuming, and have some
limitations in electrical and electrochemical performances, and
scale-up. Thereby, efficient alternative methods for the
preparation of robust, free-standing and conductive nanomembranes
with the capability of being produced in large dimensions are
highly desired.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a mechanically
robust, electrically conductive, free-standing and transparent
hybrid nanomembrane with high electrochemical capacitance that is
based on densified ultrathin carbon nanotube sheets (CNSs) and a
conducting polymer coated on the CNSs by vapor phase polymerization
(VPP), and a method for fabricating the hybrid nanomembrane.
It is a further of the present invention to provide an energy
storage electrode, such as a supercapacitor, which includes the
hybrid nanomembrane.
According to an aspect of the present invention, there is provided
a free-standing hybrid nanomembrane capable of energy storage
including carbon nanotube sheets and a conducting polymer coated on
the carbon nanotube sheets wherein the carbon nanotube sheets are
densified sheets formed by evaporating an alcohol from carbon
nanotube aerogel sheets, and the conducting polymer is coated on
the carbon nanotube sheets by vapor phase polymerization.
In an embodiment of the present invention, the carbon nanotube
sheets may consist of one or two layers.
In a further embodiment of the present invention, the conducting
polymer is preferably present in an amount of 30 to 90% by weight
and may be, for example, poly(3,4-ethylenedioxythiophene),
polyaniline, polypyrrole, polyethylene, or polythiophene.
The hybrid nanomembrane of the present invention may be
free-standing in liquid and its sheet shape is maintained at the
liquid/air interface.
In an embodiment of the present invention, the free-standing hybrid
nanomembrane has an electrochemical capacitance of 60 to 200 F/g at
10 mV/s in the potential range of -0.2 to 0.8 V, a mechanical
strength of 120 to 150 MPa, and a modulus of 8 to 18 GPa.
In an embodiment of the present invention, the carbon nanotube
sheets constituting the free-standing hybrid nanomembrane may be
formed on a transparent conducting substrate, for example, a
poly(ethylene terephthalate) (PET) substrate.
In an embodiment of the present invention, the free-standing hybrid
nanomembrane has a thickness of 60 to 120 nm.
According to another aspect of the present invention, there is
provided a method for fabricating a hybrid nanomembrane in which a
conducting polymer is coated on carbon nanotube sheets, the method
including
1) allowing an oxidant solution including Fe(III)PTS, pyridine and
butanol to flow on carbon nanotube sheets, and completely
evaporating the butanol, and
2) dropping a monomer for a conducting polymer polymerization on
areas that are not in direct contact with the carbon nanotube
sheets stained with the oxidant solution free from butanol,
followed by vapor phase polymerization in a sealed state.
In an embodiment of the present invention, the method may further
include 3) rinsing the hybrid nanomembrane to remove unreacted
oxidants after step 2).
In an embodiment of the present invention, the carbon nanotube
sheets are preferably densified sheets formed by evaporating an
alcohol from carbon nanotube aerogel sheets, and the monomer for
the conducting polymer may be, for example, ethylenedioxythiophene
(EDOT).
The concentration of the Fe(III)PTS in the oxidant solution is
preferably from 6 to 20% by weight, based on the weight of the
butanol.
The alcohol is preferably evaporated at 40 to 60.degree. C. in step
1), and the polymerization is preferably carried out at around
60.degree. C. in step 2).
According to yet another aspect of the present invention, there is
provided an energy storage electrode, such as a supercapacitor,
which includes the free-standing hybrid nanomembrane.
The robust and free-standing conductive nanomembrane of the present
invention is based on CNSs and a conducting polymer. The hybrid
nanomembrane has a sheet resistance below 200 .OMEGA./sq at a
thickness of about 66 nm and shows moderate optical transparency,
high flexibility and a bending resistance change of below 0.1%. In
addition, electrochemical capacitance of the hybrid nanomembrane is
10 times higher than that of bare CNSs due to effective
hybridization of the CNSs and the conducting polymer as well as its
area density as low as 30 .mu.g/cm.sup.2. Furthermore, the hybrid
nanomembrane is applicable to sensors, actuators, optical devices,
fuel cells as well as electrochemical capacitors.
BRIEF DESCRIPTION OF THE DRAWINGS
These and/or other aspects and advantages of the invention will
become apparent and more readily appreciated from the following
description of the embodiments, taken in conjunction with the
accompanying drawings of which:
FIG. 1 shows a) a schematic representation of a PEDOT-coated CNS
nanomembrane according to an embodiment of the present invention,
b) a scanning electron microscope image showing a morphology of a
hybrid nanomembrane, c) an atomic force microscopy image showing a
surface morphology and a height profile of a hybrid nanomembrane
with an average thickness of 66 nm on a Si wafer, in which the
thickness deviation of the hybrid nanomembrane was determined from
root-mean-square of heights, d) a photo image of a free-standing
and optically transparent hybrid nanomembrane in ethanol, and e) an
photo image of a helically twisted hybrid nanomembrane/PET
film;
FIG. 2 shows a) a stress-strain curve of a hybrid nanomembrane
according to an embodiment of the present invention, b) an
electrical resistance variation of a hybrid nanomembrane attached
on a PET film caused by curvature changes of 0 to 0.2 mm.sup.-1, in
which the plotted resistances are average values obtained from
three cycles and the inset shows flat and bent samples (Scale bar:
1 cm), and c) photo images showing shape reversibility of a hybrid
nanomembrane;
FIG. 3 graphically shows the electrochemical capacitance
performance of hybrid nanomembranes according to embodiments of the
present invention, a) cyclic voltammograms (current density versus
voltage) for hybrid nanomembranes with 1 and 2-layer CNSs and
2-layer CNSs, b) galvanostatic charge-discharge (voltage versus
time) curves for a 1-layer CNS/PEDOT nanomembrane at a current
density of 0.14 A/g and a 2-layer CNSs/PEDOT nanomembrane at a
current density of 0.16 A/g, c) specific capacitances of hybrid
nanomembranes according to different scan rates (5 to 50 mV/s) at
room temperature and 58.degree. C., and d) specific capacitances of
hybrid nanomembranes attached on a glassy carbon electrode and
hybrid nanomembranes without a glassy carbon electrode, in which
the PEDOT weight percent of all the hybrid nanomembranes was
85%;
FIG. 4 shows polarized Raman spectra of a densified 1-layer CNS, in
which a VV configuration (parallel polarization for incident light
and Raman signal) was used, with polarization parallel to
(.parallel.) or perpendicular to (.perp.) the draw direction of the
nanotube sheets;
FIG. 5 shows atomic force microscopy images showing surface
morphology and height profile, a) bare CNSs with an average
thickness of 52 nm on a Si wafer, b) a 2-layer CNS/85 wt % PEDOT
nanomembrane with an average thickness of 112 nm on a Si wafer, in
which the thickness deviation was determined from root-mean-square
of heights;
FIG. 6 shows a) electrical sheet resistance, b) area density, and
c) transmittance at a wavelength of 550 nm with different layers of
CNSs and PEDOT weight: 1-layer CNS/PEDOT nanomembranes are blue
rectangles and 2-layer CNS/PEDOT nanomembranes are red
rectangles;
FIG. 7 shows photo images showing hybrid nanomembranes attached on
a paper clip-wire in liquid and air, a) the hybrid nanomembrane was
partially immersed into ethanol, and b) the hybrid nanomembrane
passed through an ethanol/air interface without severe structural
deformation;
FIG. 8 shows cyclic voltammograms (current density versus voltage)
for bare CNSs and hybrid nanomembranes with various PEDOT
concentrations, in which the hybrid nanomembrane with 85 wt % PEDOT
showed maximum specific capacitance and the scan rate was 10
mV/s;
FIG. 9 shows Nyquist plots for 1 and 2-layer CNS hybrid
nanomembranes with 85 wt % PEDOT, in which the inset is a magnified
image in high frequency region; and
FIG. 10 shows cyclic voltammograms (current density versus voltage)
for 1-layer CNS/85 wt % PEDOT nanomembranes attached on a glassy
carbon showing rectangular shapes in high scan rates ranging from
100 to 1500 mV/s.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will now be described in detail.
The present invention provides a free-standing hybrid nanomembrane
capable of energy storage including carbon nanotube sheets and a
conducting polymer coated on the carbon nanotube sheets wherein the
carbon nanotube sheets are densified sheets formed by evaporating
an alcohol from carbon nanotube aerogel sheets, and the conducting
polymer is coated on the carbon nanotube sheets by vapor phase
polymerization.
In an embodiment of the present invention, the carbon nanotube
sheets may consist of one or two layers.
In a further embodiment of the present invention, the conducting
polymer is preferably present in an amount of 30 to 90% by weight.
The presence of the conducting polymer in an amount of less than
30% by weight increases the risk that the sheets may be torn and
causes a dramatic decrease in capacitance. Meanwhile, the presence
of the conducting polymer in an amount of more than 90% by weight
leads to a great reduction in transparency and a decrease in
capacitor efficiency per unit weight. Examples of conducting
polymers suitable for use in the hybrid nanomembrane of the present
invention include, but are not limited to,
poly(3,4-ethylenedioxythiophene), polyaniline, polypyrrole,
polyethylene, and polythiophene.
The hybrid nanomembrane of the present invention can be
free-standing in liquid and its sheet shape is maintained at the
liquid/air interface.
In an embodiment of the present invention, the electrochemical
capacitance of the hybrid nanomembrane measured at 10 mV/s in the
potential range of -0.2 to 0.8 V is from 60 to 200 F/g, which is
high when compared to that of a CNT/PEDOT film having a similar
thickness (<60 nm) (.about.33 F/g).
In an embodiment of the present invention, the hybrid nanomembrane
has a mechanical strength of 120 to 150 MPa and a modulus of 8 to
18 GPa. The mechanical strength of the hybrid nanomembrane is about
3 times higher than that of a PEDOT/PSS film with a thickness of 30
.mu.m (43 MPa). Moreover, the strength and modulus of the hybrid
nanomembrane exceed those of recently developed nanomembranes
prepared by self-assembly (elastic moduli of 1 to 10 GPa and
ultimate strengths of up to 100 MPa). This indicates that the
hybrid nanomembrane is structurally stable and is mechanically
robust enough to handle despite its nanoscale thickness.
In an embodiment of the present invention, the carbon nanotube
sheets constituting the free-standing hybrid nanomembrane may be
formed on a transparent conducting substrate, for example, a
poly(ethylene terephthalate) (PET) substrate.
In an embodiment of the present invention, the free-standing hybrid
nanomembrane has a thickness of 60 to 120 nm, which can be
considered as falling within the scope of `nanomembrane.`
The present invention also provides a method for fabricating a
hybrid nanomembrane in which a conducting polymer is coated on
carbon nanotube sheets, the method including
a method for fabricating a hybrid nanomembrane in which a
conducting polymer is coated on carbon nanotube sheets, the method
including
1) allowing an oxidant solution including Fe(III)PTS, pyridine and
butanol to flow on carbon nanotube sheets, and completely
evaporating the butanol, and
2) dropping a monomer for a conducting polymer polymerization on
areas that are not in direct contact with the carbon nanotube
sheets stained with the oxidant solution free from butanol,
followed by vapor phase polymerization in a sealed state.
In an embodiment of the present invention, the method may further
include 3) rinsing the hybrid nanomembrane to remove unreacted
oxidants after step 2).
In an embodiment of the present invention, the carbon nanotube
sheets are densified sheets formed by evaporating an alcohol from
carbon nanotube aerogel sheets and the monomer for the conducting
polymer may be, for example, ethylenedioxythiophene (EDOT).
The concentration of the Fe(III)PTS in the oxidant solution is
preferably from 6 to 20% by weight, based on the weight of the
butanol. At a concentration of less than 6% by weight, dramatic
decrease of capacitance is caused. Meanwhile, at a concentration of
more than 20% by weight, a great reduction in transparency, an
increase in thickness, and a decrease in capacitor efficiency per
unit weight are caused.
In the method of the present invention, the alcohol is preferably
evaporated at 40 to 60.degree. C. At a temperature lower than
40.degree. C., it takes a long time to completely evaporate the
alcohol. In step 2), the polymerization is carried out at
60.degree. C. Depending on the polymerization temperature, the
evaporation rate of ethylenedioxythiophene (EDOT) is varied, and as
a result, the coating conditions may be varied.
The free-standing hybrid nanomembrane of the present invention is
applicable to energy storage electrodes, such as supercapacitors,
due to its very high capacitance.
The present invention will be explained in detail with reference to
the following examples. These examples are provided to assist in
further understanding of the invention and are not intended to
limit the scope of the invention.
EXAMPLES
Example 1: Fabrication of Hybrid Nanomembranes
In this example, transparent and conductive hybrid membranes were
fabricated that are stable in liquid or at the liquid/air
interface. CNSs densified by an oxidant solution were polymerized
with poly(3,4-ethylenedioxythiophene) (PEDOT) employing (VPP), as
shown in FIG. 1a. The stacked CNSs by VPP were optically
transparent, had a thickness of below 100 nm, and consisted of one
or two-layer carbon nanotube aerogel sheets.
Because the high electrical conductivity, optical transparency,
chemical stability and electrochemical catalytic property, PEDOT
has been considered for applications including
electrochemical/mechanical sensors, transparent electrodes, and
fuel cell membranes. Therefore, in this example, PEDOT was employed
to fabricate transparent hybrid membranes.
First, aligned arrays of carbon multi-walled nanotubes (MWNT
forests) were grown on a Si wafer using a conventional chemical
vapor deposition method. Iron(III)p-toluenesulfonatehexahydrate
(Fe(III)PTS, M.sub.w: 677.52), pyridine (anhydrous, 99.8%),
1-butanol (for molecular biology, .gtoreq.99%), and
3,4-ethylenedioxythiophene (EDOT) monomer (97%) were purchased from
Sigma-Aldrich (USA). 2N sulfuric acid solutions (1M
H.sub.2SO.sub.4) were purchased from Daejung chemicals (South
Korea).
0.1 mL of pyridine was added to a 20 wt % Fe(III)PTS/butanol
solution and dissolved with stirring using a magnetic bar or by
sonication to prepare a Fe(III)PTS/pyridine/butanol solution. The
Fe(III)PTS/pyridine/butanol solution was diluted with butanol to
prepare solutions having various concentrations ranging from 1 to 8
wt % oxidant. Each of the diluted solutions were allowed to flow on
slideglass, on which one or two superimposed sheets of well-aligned
carbon multi-walled nanotubes (MWNT) had been placed and dried in
an oven at 60.degree. C. to completely evaporate the butanol. The
ethylenedioxythiophene (EDOT) monomer was dropped on both the sides
around the MWNT CNSs stained with the oxidant solution, and then
the sample was sealed. Vapor phase polymerization was carried out
at 60.degree. C. for 1 hour. After polymerization, the nanomembrane
was thoroughly rinsed several times in ethanol to remove unreacted
oxidants.
The surface morphology and height profiles of the hybrid
nanomembranes were taken using scanning electron microscopy
(Hitachi S4700, Japan) and atomic force microscopy (Park Systems
XE-100, South Korea). Galvanostatic, potentiostatic, and
electrochemical impedance measurements were performed employing a
Gamry instrument (USA). Two-probe sheet resistance measurements
were performed using a digital multimeter (Fluke Corporation, Model
187, USA), and two-probe bending resistance changes were measured
using a home-made bending-recovery device.
FIG. 1 shows PEDOT/CNSs hybrid nanomembranes prepared by VPP. In
FIG. 1, a) is a schematic representation of a PEDOT-coated CNS
nanomembrane. b) is a scanning electron microscope image showing a
morphology of a hybrid nanomembrane. c) is an atomic force
microscopy image showing a surface morphology and a height profile
of a hybrid nanomembrane with an average thickness of 66 nm on a Si
wafer. The thickness deviation of the hybrid nanomembrane was
determined from root-mean-square of heights. d) is a photo image of
a free-standing and optically transparent hybrid nanomembrane in
ethanol. e) is an photo image of a helically twisted hybrid
nanomembrane/PET film. The hybrid nanomembrane was directly
attached on a PET film by VPP without transfer. All samples were
prepared from densified 1-layer CNS, and loading of PEDOT in the
hybrid nanomembranes was 85 wt %.
FIG. 2 shows high mechanical and structural robustness of a hybrid
nanomembrane of the present invention. In FIG. 2, a) shows a
stress-strain curve of a hybrid membrane. b) shows an electrical
resistance variation of a hybrid nanomembrane attached on a PET
film caused by curvature changes of 0 to 0.2 mm.sup.-1. The plotted
resistances are average values obtained from three cycles. The
inset shows the flat and bent samples (Scale bar: 1 cm).
In FIG. 2, c) shows photo images showing shape reversibility of a
hybrid nanomembrane: free-standing nanomembrane in liquid (left),
nanomembrane at liquid/air interface (middle), and collapsed
nanomembrane in air (right). The arrows indicate that shape
recovery of a collapsed nanomembrane is possible by re-immersing
the membrane in liquid. PEDOT weight percent was 85%, and 1-layer
CNS was used for the hybrid nanomembrane preparation.
FIG. 3 shows the electrochemical capacitance performance of hybrid
nanomembranes. In FIG. 3, a) shows cyclic voltammograms (current
density versus voltage) for hybrid nanomembranes with 1 and 2-layer
CNSs and 2-layer CNSs. b) shows galvanostatic charge-discharge
(voltage versus time) curves for a 1-layer CNS/PEDOT nanomembrane
at a current density of 0.14 A/g and a 2-layer CNSs/PEDOT
nanomembrane at a current density of 0.16 A/g. c) shows specific
capacitances of hybrid nanomembranes according to different scan
rates (5 to 50 mV/s) at room temperature and 58.degree. C. d) shows
specific capacitances of hybrid nanomembranes attached on a glassy
carbon electrode and hybrid nanomembranes without a glassy carbon
electrode. The PEDOT weight percent of all the hybrid nanomembranes
was 85%.
Densified nanoscale thin CNSs played a role as a highly conductive
and transparent matrix. Previously, the preparation of conductive
PEDOT nanomembranes on insulating Goretex substrates required
additional gold coating before PEDOT polymerization for a stable
electrical connection between PEDOT polymers. However, CNSs used in
the present invention enable direct polymerization of PEDOT for
conductive nanomembranes due to the well-aligned carbon nanotubes
with high electrical conductivity. Aligned structures of CNTs in
densified CNS by butanol used in the present invention were
confirmed from polarized Raman spectroscopy (FIG. 4).
This direct coating without gold wiring is a very significant
factor to obtain high specific electrochemical capacitance and low
area density. Such CNSs were densified from 1 or 2-layer carbon
nanotube aerogel sheets by the flow of a dilute oxidant solution (8
wt % oxidant in butanol), and small amount of oxidant deposition
generated ultrathin layer structures with distinct carbon
nanotube/PEDOT fibrous morphology after VPP (FIG. 1b).
The PEDOT weight percent of hybrid nanomembranes were controllable
by oxidant concentration in solutions, and 85 wt % PEDOT was coated
on 1-layer CNS using an 8 wt % oxidant solution. The resultant
hybrid nanomembranes with 85 wt % PEDOT had thickness of average
.about.66 nm as shown in FIG. 1c. The average thickness of 1-layer
CNS/85 wt % PEDOT nanomembrane was larger than that of 1-layer CNS
densified by butanol (.about.52 nm) and smaller than that of
2-layer CNS/85 wt % PEDOT nanomembrane (.about.112 nm) (FIG.
5).
Electrical conductance of hybrid nanomembranes was enhanced with
the increase of PEDOT weight percent (FIG. 6a). The hybrid
nanomembrane consisting of 85 wt % PEDOT and 1-layer CNS had a
sheet resistance of 312 W/sq, and the value was three times lower
than that of 1-layer CNS densified by butanol (958 W/sq). The
dramatic decrease of sheet resistances is attributed to nanoscale
thin PEDOT polymers filling spaces between individual CNTs as shown
in FIGS. 1b and 1c. The sheet resistance of hybrid nanomembranes
was further decreased when 2-layer CNSs were used as a substrate
for PEDOT polymerization. The sheet resistance was then 163 W/sq at
85 wt % PEDOT coating. The sheet resistance was half that of the
1-layer CNS/85 wt % PEDOT sample. The reduced sheet resistance is
related to the increase of area density (FIG. 6b), since the area
density of the 2-layer CNS/85 wt % PEDOT nanomembrane (29
.mu.g/cm.sup.2) was almost double compared to that of 1-layer
CNS/85 wt % PEDOT nanomembrane (15 .mu.g/cm.sup.2). Although
multi-layer stacked CNS samples (over 3 layers) can further
increase the sheet conductance of the hybrid nanomembranes, the
thickness and the area density of hybrid membranes were also
increased, resulting in a loss of their transparency.
The hybrid nanomembrane peeled off on a glass slide after VPP was
optically transparent and free-standing in ethanol (FIG. 1d). The
transmittance spectra at wavelength of 550 nm were obtained from
hybrid nanomembranes with different PEDOT loading (FIG. 6c). The
transmittance of the hybrid nanomembrane (85 wt % PEDOT and 1-layer
CNS) was 56% and this value was 18% lower than that of the 1-layer
CNS densified by butanol (74%). The reduced transmittance of the
hybrid membrane is due to decreased porosity between the CNTs
caused by PEDOT polymerization and also due to the increased
membrane thickness. However, the hybrid nanomembrane still has
reasonable optical transparency (FIGS. 1d and 1e).
The hybrid nanomembrane can be directly prepared on a flexible and
transparent substrate such as poly(ethylene terephthalate) (PET)
without the need for a membrane transfer process. The flexibility
of the hybrid membrane/PET film and good adhesion between the
hybrid nanomembrane and PET substrate enabled the laminate to
survive twisting into a helical shape without severe structural
damage (FIG. 1e).
To investigate mechanical and structural robustness of hybrid
nanomembranes, various tensile and bending tests in liquid and at a
liquid/air interface were carried out. FIG. 2a shows a
stress-strain curve of a hybrid nanomembrane consisting of 1-layer
CNS/85 wt % PEDOT. The density normalized mechanical strength and
modulus of hybrid nanomembranes were 51.+-.3 MPa/(g/cm.sup.3) and
5.1.+-.2.9 GPa/(g/cm.sup.3), respectively. When mechanical
properties were calculated using the average thickness of the
hybrid nanomembranes (obtained from AFM), the mechanical strength
and modulus were 135.+-.8 MPa and 12.6.+-.5.3 GPa, respectively.
This mechanical strength was .about.3 times higher than that of
PEDOT/PSS film with thickness of 30 mm (43 MPa). Moreover, the
strength and modulus of 1-layer CNS/85 wt % PEDOT nanomembrane
exceeded those of the recently developed nanomembranes prepared by
self-assembly (elastic moduli of 1-10 GPa and ultimate strengths of
up to 100 MPa). (Appl. Phys. Lett. 2005, 86, 121912)
The electrical resistance changes of hybrid nanomembranes with
1-layer CNS/85 wt % PEDOT during bending of the hybrid/PET laminate
were also investigated using a home-made bending device (FIG. 2b).
The electrical resistance changes were below 0.1% up to a bend
curvature of 0.2 mm.sup.-1. Generally, conducting polymers are very
brittle, particularly in nanoscale thin films. The brittleness
often generates dramatic increases of resistance by deformation
such as bending. Almost complete recovery of bending resistances in
hybrid nanomembranes reflects that CNTs composing CNSs provide
flexibility without structural damage during deformation.
CNSs have difficulties in maintaining their sheet shape in liquid
without a supporting substrate because of a collapsing phenomenon
induced by their small buckling threshold and van der Waals
interactions between individual CNTs. Moreover, CNSs that collapse
at a liquid/air interface do not recover their original shape when
they are immersed in liquid again. Unlike CNSs, the hybrid
nanomembranes of the present invention completely recovered their
shape when there were wetted again in liquid (before complete dry)
although the hybrid nanomembranes were collapsed by surface tension
at liquid/air interface (FIG. 2c). It was also confirmed that the
hybrid nanomembranes attached on paper clip-wires pass through
ethanol/air interface without structural deformation (FIG. 7). This
stability indicates that the hybrid nanomembranes are very robust
and strong despite their nanoscale thickness and low area density
of .about.15 .mu.g/cm.sup.2.
Example 2: Determination of Transparent and Hybrid Nanomembranes'
Performance for Supercapacitors
For determining transparent and hybrid nanomembranes' performance
for supercapacitors, the electrochemical capacitance of
three-electrode cells consisting of hybrid nanomembranes (working
electrode), Pt mesh (counter electrode), and Ag/AgCl (reference
electrode) in 1M sulfuric acid was measured, as shown in FIG. 3.
The present inventors focused on hybrid nanomembranes with 85 wt %
PEDOT because this weight ratio gave the highest electrochemical
capacitance (FIG. 8). FIG. 3a shows the results of typical CV
measurements of hybrid nanomembranes and 2-layer CNS. The
rectangular shapes of the CV curves in the potential range of -0.2
to 0.8 V indicate a predominantly capacitive behavior of the hybrid
nanomembranes.
The specific capacitance was obtained using
(I.sub.a+|I.sub.c|).times..DELTA.t/(2 m.DELTA.V), where I.sub.a and
I.sub.c are the anodic and cathodic voltammetric current on the
anodic and cathodic scans, .DELTA.t is scanning time, m is the mass
of the hybrid nanomembrane, and .DELTA.V is the potential range of
the CV. The 1 and 2-layer CNS/85 wt % PEDOT nanomembranes had
electrochemical capacitance values of 62 F/g and 84 F/g at a scan
rate of 10 mV/s, respectively. These values were .about.8 and
.about.10 times higher than that of 2-layer CNS. The high
capacitance is related to high pseudo-capacitance of PEDOT and
effective hybridization of PEDOT and individual CNTs composing
CNSs.
The galvanostatic charge/discharge curves for hybrid nanomembranes
are shown in FIG. 3b. The specific capacitance was also calculated
from the discharge curves according to
(I.times.t.sub.d)/(m.times.V), where I is the discharge current,
t.sub.d is the discharging time, m is the weight of the active
material, and V is potential range. 1 and 2-layer CNS/85 wt % PEDOT
nanomembranes had capacitance values of 89 and 106 F/g, which were
obtained at current densities of 0.14 and 0.16 A/g,
respectively.
The capacitance values obtained from the galvanostatic
charge/discharge curves were 20-30% higher than those calculated
from CV curves. IR drops of 1 and 2-layer CNS/85 wt % PEDOT
nanomembranes were 70 and 10 mV, respectively. The smaller IR drop
of the hybrid nanomembrane with 2-layer CNS was attributed to its
lower equivalent series resistance (ESR) value, and the ESR values
were confirmed from Nyquist plots of hybrid nanomembranes (1-layer
CNS/85 wt % PEDOT: 102 Ohm, 2-layer CNS/85 wt % PEDOT: 49 Ohm)
(FIG. 9).
Generally, electrochemical capacitance increases with the increase
of electrolyte temperature. Experiments were carried out at
58.degree. C. because electrical properties of the PEDOT are
maintained until 60.degree. C. Temperature of an electrolyte (1M
H.sub.2SO.sub.4) was slowly elevated for 3 hours to reach the
equilibrium state. At 58.degree. C. the specific capacitance of
1-layer CNS/85 wt % PEDOT nanomembrane measured at 10 mV/s was
.about.72 F/g, and this value was .about.50% higher than that
measured at room temperature at the same scan rate (.about.48 F/g)
(FIG. 3c). The energy density and the power density of hybrid
nanomembrane were .about.20 Wh/kg and .about.1950 W/kg at
58.degree. C., respectively.
Fast charging response was achieved by laminating the hybrid
nanomembrane on a highly conducting current collector, such as
glassy carbon. Without the current collector, the free-standing
hybrid nanomembranes showed a diminishing capacitance with
increasing voltage scan rate (FIG. 3c).
In contrast, the nanomembranes attached to the glassy carbon
electrode maintained their high electrochemical capacitance in scan
rates between 5 and 50 mV/s without dramatic decrease of
capacitance (FIG. 3d).
The 1-layer CNS/85 wt % PEDOT nanomembrane attached on a glassy
carbon also showed rectangular shapes of CV curves in high scan
rates ranging from 100 to 1500 mV/s (FIG. 10). At 5 and 1500 mV/s,
the specific capacitances of hybrid nanomembrane were .about.64 and
.about.50 F/g, respectively, and the hybrid nanomembrane on a
current collector showed decrease of .about.22% in capacitance.
Thereby, the hybrid nanomembranes can have superior capacitance
performance on transparent and conductive electrodes such as indium
tin oxide glass, suggesting that they can be used as transparent
supercapacitors.
* * * * *
References